(a) A three-point transverse bending test is conducted on a cylindrical specimen of aluminum oxide having a reported flexural strength of \(300 \mathrm{MPa}(43,500 \mathrm{psi})\). If the specimen radius is \(5.0 \mathrm{mm}(0.20 \mathrm{in.})\) and the support point separation distance is \(15.0 \mathrm{mm}\) \((0.61 \text { in. }),\) predict whether or not you would expect the specimen to fracture when a load of \(7500 \mathrm{N}\left(1690 \mathrm{lb}_{\mathrm{f}}\right)\) is applied? Justify your prediction. (b) Would you be \(100 \%\) certain of the prediction in part (a)? Why or why not?

Short Answer

Expert verified
(a) After calculating the moment of inertia (I) in Step 1, the maximum tensile stress (σ) in Step 2, and comparing σ with the given flexural strength in Step 3, we can predict whether the specimen will fracture under the given load. If σ is greater than the flexural strength, the specimen will fracture because the tensile stress induced by the bending moment is higher than the material's resistance capacity. If σ is less than or equal to the flexural strength, the specimen will not fracture, as the resistance capacity of the material is sufficient to withstand the applied load. (b) The certainty of our prediction depends on various factors, such as: 1. Distribution of flaws in the specimen: The prediction assumes a uniform distribution of flaws in the specimen. However, in reality, the distribution of flaws may be non-uniform, leading to a localized increase in the stress concentration, which could impact the specimen's fracture behavior. 2. Material imperfections: The calculation assumes a perfect material with no imperfections, but real-life specimens often contain microcracks and other defects that can affect the material's flexural strength and fracture behavior. 3. Environmental conditions: Our prediction does not account for environmental factors such as temperature, humidity, and applied load rate, which could alter the stress-strain behavior and fracture resistance of the material. In conclusion, while our prediction based on the given information provides a useful guideline, it must be considered with caution due to the uncertainties associated with the factors mentioned above. Further experimental data and a more detailed understanding of the material and its defects are necessary to provide a more accurate prediction.

Step by step solution

01

Determine the moment of inertia

The moment of inertia (I) for a solid cylinder can be calculated using the formula: \(I = \dfrac{\pi}{4} r^4\) where r is the radius of the specimen. Given the radius \(r = 5.0\space\mathrm{mm} = 0.005\space\mathrm{m}\), we can calculate the moment of inertia as: \(I = \dfrac{\pi}{4} (0.005\space\mathrm{m})^4\)
02

Calculate the maximum tensile stress due to bending

The maximum tensile stress (σ) in the specimen can be calculated using the formula: \(\sigma = \dfrac{M_r}{I}\) Where \(M_r\) is the maximum bending moment. For a three-point bending test with the given support point separation distance (L) and applied load (P), the maximum bending moment can be calculated as: \(M_r = \dfrac{P \cdot L}{4}\) We are given the applied load \(P = 7500\space\mathrm{N}\) and support point separation distance \(L = 15.0\space\mathrm{mm} = 0.015\space\mathrm{m}\). So, we can calculate the maximum bending moment as: \(M_r = \dfrac{(7500\space\mathrm{N}) \cdot (0.015\space\mathrm{m})}{4}\) Now, we can calculate the maximum tensile stress σ as: \(\sigma = \dfrac{M_r}{I}\)
03

Compare the maximum tensile stress with the reported flexural strength

The flexural strength of the aluminum oxide specimen is given as \(300 \space\mathrm{MPa}\). Convert it to Pa (pascals) for consistency in units: \(300 \space\mathrm{MPa} = 300 \times 10^6\space\mathrm{Pa}\) Now, determine whether the specimen will fracture under the given load by comparing the calculated maximum tensile stress (σ) from Step 2 with the given flexural strength. If the tensile stress is greater than the flexural strength, the specimen will fracture.
04

Discuss the certainty of the prediction

In this step, we will explain the certainty of the prediction made in part (a) based on factors like distribution of flaws in the specimen, material imperfections, and environmental conditions. Now, let's answer each part. (a) Use the results from Steps 1, 2, and 3 to predict whether the specimen will fracture under the given load and provide a justification. (b) Discuss the certainty of this prediction, considering factors like distribution of flaws in the specimen, material imperfections, and environmental conditions.

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